Web servers return HTTP headers with every response, and usually those headers specify how long the response can be cached for. Proxy servers read those headers and make a decision whenever they see the same request again -- whether to propagate that request to the server again, or to simply return the cached copy of the response.
You can modify your server's configuration so that it the next time it tells the proxy server not to cache pages. However, some proxy servers are mis-configured or broken, and will cache pages that they are not supposed to cache.
For those cases, one ugly solution that works is to give your JS and CSS files new names whenever you change them. For example, if your index.html file includes index.css and index.js, and you make a change to index.js, you can save the changed file as index.2.js and change the tag in your index.html file to point to index.2.js from now on.
That's a bit drastic, but it works. A simpler solution to start with is to refresh your page using Shift-F5 rather than just F5 (in your browser). That tells the browser to force a refresh of all cached pages, whenever possible.